[DFTB-Plus-User] Help required: Geometry optimisation under PBC // how to consider the resultantpresure?

[rsaha at icredd.hokudai.ac.jp]

Hello All,

I am optimising a structure under PBC. The structure is a helix and the 
repetitive unit is in the cell (650 atoms/cell). I gave a=c= very large 
length, so no possible interaction from that side, and optimising only b 
unit length. I am using the following input lines,
    LatticeOpt = Yes 
    FixAngles = Yes             # Fix angles between lattice vectors (
cubic cell)
    FixLengths = {Yes No Yes}   # Allow only latt. vector 2 or b to 
change

The structure is optimised with satisfactory features (I am new to DFTB+)
. The total energy of the optimised geometry is -799.9150983040 H. But I 
found that the pressure is 0.452052E+06 Pa (4.5 atm). 

I am curious to know about this resultant pressure. Is the reultant 
pressure value can be any indication for the good or bad structure.

I have tried with MaxForceComponent = 1.0E-8 in another job, just to 
check the resultant pressure variation. The result show an increase in 
the total energy -799.8832048350 H and Pressure 0.555629E+07 Pa (54.8 
atm). 

So it seems the second job gave higher total energy as well as higher 
pressure as compared to the first one.

So my queries are, 
1) How to check that the optimisation gave me a good geometry?
2) What are the criteria that I should check for the optimisation under 
PBC?
3) Should I bother about the resultant pressure and how to consider it 
as good pressure for the optimisation?  
4) If I am missing something please let me know.

Thank you all for your time aand considerations.
Sincerely
Ranajit